Almost a decade after Middle Eastern terrorists with expired visas attacked the U.S., the government has failed to implement a security measure to adequately track foreigners like them who enter the country legally but never leave.As a result nearly half of the nation’s estimated 12 million illegal immigrants actually entered the U.S. legally but overstayed their visa, according to a new federal report. That’s because the agency responsible for keeping the nation safe—the Department of Homeland Security—can’t keep track of immigrants who remain in the U.S. after their visas expire.This clearly creates a huge national security issue because terrorists can plot more attacks from within. In fact, dozens of foreigners convicted of terrorism since the 2001 attacks had overstayed their visas, according to the report, which was published by the investigative arm of Congress known as the Government Accountability Office (GAO).The GAO launched its probe after learning from an independent study that 4 to 5.5 million immigrants had entered the country legally and overstayed their authorized periods of admission. In the course of the probe, GAO investigators interviewed a number of high-ranking Homeland Security officials, visited various field offices and reviewed a mountain of documents.They found that the Department of Homeland Security’s program to identify visa violators by comparing entry and exit data is inefficient and can only process around half of the potential overstays it detects. Currently, the system has a backlog of about 1.6 suspected foreigners who remained in the U.S. past their visa period, the probe found. Some could very well be Islamic extremists.This may seem unbelievable, especially since most of the 9/11 hijackers capitalized on this weakness in the system to plan the murder of thousands of Americans. The massive loophole is precisely why the Homeland Security agency (Immigration and Customs Enforcement—ICE) in charge of tracking down foreigners who overstay visas is well funded by Congress to the tune of 70-plus million dollars annually.The agency doesn’t dispute the findings or the recommendations made by GAO investigators, who suggest establishing a “time frame for completing overstay enforcement planning” and “performance measurement mechanisms,” among other things. In other words don’t wait another decade to implement security measures that actually work.